The family of materials known as zeolites constitute a large group of silicates having appreciable void volume within their structures. In the ideal state they may be viewed as built from corner shared SiO4 tetrahedral building units which form a large range of architectures comprising cavities, channels and cages. In the pure silica forms the structures are charge neutral frameworks stuffed with either neutral molecules, usually water or other neutral solvent molecules, or salt pairs, such as NaCl. These pure silica forms have been designated “clathrasils” or “zeosils” (Liebau et al., Zeolites, v. 6, 373, 1986). More commonly Al substitutes for some of the silica, in which case the framework possesses a net negative charge which is balanced by “exchangeable” cations-commonly those of Groups 1 and 2 of the Periodic Table (Kirk-Othmer Encyclopedia of Chemical Technol., J. Wiley (New York), v. 8, 94, 1965). However, numerous substitutions are now recognized as being possible both in the Si framework substituents and the exchangeable cations, as demonstrated in much of the recent art. A major expansion of these structural types has been achieved with the recognition that AIPO4 has many structures beyond the well known silica analogues of quartz-tridymite—cristobalite (Flanigen et al., Proc. 7th Intl. Zeolite. Conf., Ed. Murakami et al., Kodansha/Elsevier (Tokyo), p. 103, 1985). Many zeolites occur as minerals (Tschemich, “Minerals of the World”, Geoscience Press (Phoenix, Ariz.) 1992), some of which have no synthetic counterparts. Similarly many synthetic zeolites have no naturally occurring counterparts. The large number of existing known structures has been reviewed by Meier and Olson (“Atlas of Zeolite Structures”, Butterworths-Heinemann Press (London), 1992). The unique catalytic, sorption and ion-exchange properties of these zeolite “molecular sieves” have been utilized in many industrial and environmental processes, and numerous consumer products. (As reviewed in the periodic Proceedings of the International Zeolite Conferences).
There are a large number of synthetic methods for producing zeolites, well illustrated in the patent literature and reviewed by Barrer (in “Hydrothermal Chemistry of Zeolites”, Academic Press (London), 1982), Breck (in “Molecular Sieve Zeolites”, J. Wiley (New York), 1974) and Jacobs and Martens (in “Synthesis of High Silica Alumino-silicate Zeolites.”, Elsevier (Amsterdam), 1987). Reactants may be general or specific and typical reaction conditions are below about 250.degree. C. and 50 bars pressure. The primary solvent is usually water, but others, such as ammonia (e.g., U.S. Pat. No. 4,717,560) and organic liquids (e.g., U.S. Pat. No. 5,160,500), have also been used. Methods for controlling the zeolite type produced, and its composition, include “seeds” as nucleation centers and organic molecules (frequently alkylammonium salts) as structural “templates”.
The prior art includes two standard methods for materials processing for either crystallization or precipitation. The first is the standard autoclave crystallization process using commercially available equipment in a batch operation. This is the preferred approach to crystallizing microporous and mesoporous materials. The reaction mixture is stirred to assure uniform composition of the product. The finished product is typically washed, sent through a filtration system, and then dried for further processing. A second approach is a continuous precipitation process, producing a product that again requires filtration prior to further handling.
The present invention concerns a novel process for the synthesis of zeolites, aluminophosphates, and mesoporous solids. More precisely, the present invention describes a unique two-phase synthesis technique of a zeolite, microporous material or macroporous material by first crystallizing a crystalline phase precursor and then introducing a recrystallization agent to recrystallize the precursor into a second different crystalline phase
Another family of crystalline microporous compositions known as molecular sieves, which exhibit the ion-exchange and/or adsorption characteristics of zeolites are the aluminophosphates, identified by the acronym AlPO4, and substituted aluminophosphates as disclosed in U.S. Pat. Nos. 4,310,440 and 4,440,871. U.S. Pat. No. 4,440,871 discloses a class of silica aluminophosphates, which are identified by the acronym SAPO and which have different structures as identified by their X-ray diffraction pattern. The structures are identified by a numerical number after AlPO4, SAPO4, MeAPO4 (Me=metal), etc. (Flanigen et al., Proc. 7th Int. Zeolite Conf., p. 103 (1986) and may include Al and P substitutions by B, Si, Be, Mg, Ge, Zn, Fe, Co, Ni, etc. The present invention is a new molecular sieve having a unique framework structure.
ExxonMobil and others extensively use various microporous materials, such as faujasite, mordenite, ZSM-5, MCM-41 and MCM-68 in many commercial applications. Such applications include reforming, cracking, hydrocracking, alkylation, oligomerization, dewaxing and isomerization. Any new or modified material has the potential to improve the catalytic performance over those catalysts presently employed.
Since zeolites are metastable compounds, the nature of the zeolite formed from a gel depends not only on conventional thermodynamic parameters (overall composition of the gel, temperature, pressure) but also on kinetic factors linked to the reactivity of the gel. This gel reactivity, which partially determines the degree of supersaturating achieved in the liquid phase, is a function of the method used to prepare the gel, the nature of the starting materials used, and the nature of any materials added during synthesis. In conventional synthesis of molecular sieves, the gel composition is defined at the very early state when the synthesis gel is prepared. However, there has not been an intentional perturbation of the gel composition after a first crystal phase has formed in the gel.
It has long been known to use an existing zeolite as the raw material for a new synthesis gel. See for example U.S. Pat. No. 6,080,382 or U.S. Pat. No. 5,935,551. A disadvantage to this approach is that it requires processing of the initial crystalline phase. Such processing involves separation of the crystalline product from the reaction gel through mechanical means such as filtration. The crystalline materials are then washed, possibly ion exchanged and possibly calcined to remove organic structure directing agents used in the initial crystallization. These auxiliary steps dramatically increase the cost of using zeolites or other crystalline phases as raw materials for use in producing other crystalline materials. The method described herein does not require separation of the initial crystalline phase or any additional processing of it as a raw material. It allows for the properties of the initial raw materials to be incorporated into the final, desired crystalline phase.
Similarly, it has been well known to introduce a zeolite during the synthesis process as a seed to start a synthesis of that type of zeolite. See, for example, U.S. Pat. No. 6,324,200 B1. There are also references to using seeds of one type of zeolite to facilitate the crystallization of a different, unrelated zeolite. However, the quantity of seeds is typically <5% by weight, perhaps as high as 10% by weight. In the aforementioned prior art, the reactants are all added prior to any hydrothermal synthesis, including the zeolite precursor, seeds, and any templates.
Another well-known process is the introduction of seeds to promote the crystallization of microporous and mesoporous materials. In that process, a beta zeolite is used as a seed in the original preparation with the goal of substituting one element for another in the zeolite crystal. See, for example, U.S. Pat. No. 5,972,204 or 6,103,215.
Some prior art has considered adding materials to the preparation during the synthesis process. In a standard example of a well known technique Benazzi, et al., teach in U.S. Pat. No. 5,695,735 a technique of continuously adding an acidic chemical agent to promote the condensation (and later crystallization) of MFI, OFF and beta zeolites. Wu, et al., teach in U.S. Pat. No. 5,389,358 a method of adding a reagent in a slow and controlled manner during the hydrothermal synthesis in order to purify and promote the original crystal structure. Neither of these patents teaches a method to recrystallize an initial crystalline phase after it has formed during the hydrothermal synthesis process to achieve a second crystalline phase. The current invention is significantly different from the prior art in that it teaches a method of perturbing the original crystal structure after it has completely formed in the hydrothermal synthesis process.
One type of material of interest for use with the current invention is ferrierite. Ferrierite is a natural occurring and well known zeolite characterized as layers of 5-rings which condense to form a 2-dimensional pore structure comprised of intersecting 8 and 10 membered ring channels. Synthetic forms of ferrierite are also well known in the literature.
Prior art suggests that conversion between zeolites, can be effected only by converting a zeolite with a relatively lower framework density to one of a relatively higher framework density (U.S. Pat. No. 6,436,364). The method disclosed herein demonstrates that with the proper manipulation of the synthesis process, zeolites can be converted to other zeolites with either a higher or a lower framework density. Examples described herein will demonstrate conversion of Ferrierite to MCM-68 or beta as well as beta to MCM-68. An example of discussions on energetics, at least for high silica zeolites, can be found in: Eric C. Moloy et al, “High-silica zeolites: a relationship between energetics and internal surface areas “Micro Meso Mat., Vol 54, 1-2, 1 Jul. 2002, Pages 1-13.
ZeoliteFramework Density, T Atoms/1000 ÅFerrierite17.7MCM-6816.6Beta15.3ZSM-1218.2